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[cem_devecioglu]

in addition to above (MAF check), I would also recommend 3 checks..
 
cambelt timing, DIS pack condition(there are some tests in youtube that you can apply yourself also)  and very close inspection of head gasket area both rear and front (above the alternator is a candidate).. 
 
ps: dont trust compression tests alone as mostly fire rings stay intact.. and coolant loss can be minimal.. and mileage not that important , as time with old coolant also kills the gaskets..

[minerwilly]

Keep em coming people. This is all starting to sound like great advice.

Could the head gaskets be responsible for this if they were failing?

I'll get all of these bits looked at as soon as I can. Hopefully if I can get past the initial issue of getting my car working, I might actually become a contributor on here instead of an advice leech

I'll report back when I have more information.

Thanks again everybody.

[cem_devecioglu]

Could the head gaskets be responsible for this if they were failing?

may be yes, may be no but I can bet , you wont wait long

[Rods2]

Is the going into limp mode consistent or does it happen randomly. If randomly it could be a failing crank sensor. I had this with the autobox occasionally going into limp mode as it was failing and it then started cutting the engine out when hot. I did not get any engine management codes when this happened.

Fitting a new crank sensor, which cost just over £50 with a trade card cured my problems. Only use a genuine Vauxhall sensor. As you have a 2.5l, I know a member had issues where there are two types of sensor for this engine, but they produced a guide which is in the maintenance guides section.

[dbdb]

I'm surprised the cats went at 80K/15 years, my 98 v6 had fine cats at 200K  when it was scrapped last year on other issues.  So presumably there is a causing problem. 

Do check the simple and cheap things first eg fuel filter.

Also surprised there are no error codes especially as the EML did come on.  Maybe check it is all working by forcing a fault eg disconnect a spark plug lead for a very short period.

[dbug]

Disconnecting a plug lead on a 2.5V6 won't necessariy throw a fault code

[andy_driver]

i had a problem once with the ecu coolent temp sensor the car would run ok then all of a sudden it over rev and over fuel and cut tempermental it was fitted a new sensor and it was on the other hand the past few months my car was really lumpy on idle and fuel wasnt great at all and sounded rough and slow every so slighty what i found was that after unblocking my heater matrix the problem was cured because were the ecu temp sensor sits in the coolent bridge the outlet pipe that comes out of the matrix goes back to the coolent bridge the temp sensor was getting proper hat water in stead of look warm getting wrong signal in turn cause low power the car is running great and fuel consumption is better cause the sensor is sending the right signals before the ecu was thinking the car was cold and still giving out loads of fuel how does the heater feel in your car .

[dbdb]

Disconnecting a plug lead on a 2.5V6 won't necessariy throw a fault code

P0300 Random/multiple cylinder misfire detected
P0301 Cylinder 1 Misfire Detected
P0302 Cylinder 2 Misfire Detected
P0303 Cylinder 3 Misfire Detected
P0304 Cylinder 4 Misfire Detected
P0305 Cylinder 5 Misfire Detected
P0306 Cylinder 6 Misfire Detected
No fault will always throw a code but with OBDII misfire detection is pretty good, primary info is from the crank sensor. 

[dbug]

Disconnecting a plug lead on a 2.5V6 won't necessariy throw a fault code

P0300 Random/multiple cylinder misfire detected
P0301 Cylinder 1 Misfire Detected
P0302 Cylinder 2 Misfire Detected
P0303 Cylinder 3 Misfire Detected
P0304 Cylinder 4 Misfire Detected
P0305 Cylinder 5 Misfire Detected
P0306 Cylinder 6 Misfire Detected
No fault will always throw a code but with OBDII misfire detection is pretty good, primary info is from the crank sensor.

Above are codes for Mototronic ME 3.11 which is fitted to dbw motors (2.6 & 3.2V6) - the 2.5 and 3.0 V6 uses the Mototronic 2.8.1 - different codes and misfires not recognised or coded

[dbdb]

Above are codes for Mototronic ME 3.11 which is fitted to dbw motors (2.6 & 3.2V6) - the 2.5 and 3.0 V6 uses the Mototronic 2.8.1 - different codes and misfires not recognised or coded

They are OBDII codes, they are industry standard, mandatory for all cars from 1996 (US) and Jan 2001 (Europe).  All cars meaning every make sold.  Not proprietary to Motomoronic let alone to certain models of Motromoronic.   

[dbug]

Read the maintenance guides mate

Non dbw Omegas are not OBDII compliant!!

If you don't believe me look here at TBs post #1 http://oldsite.omegaowners.com/forum/YaBB.pl?num=1176839367/0

[cem_devecioglu]

i had a problem once with the ecu coolent temp sensor the car would run ok then all of a sudden it over rev and over fuel and cut tempermental it was fitted a new sensor and it was on the other hand the past few months my car was really lumpy on idle and fuel wasnt great at all and sounded rough and slow every so slighty what i found was that after unblocking my heater matrix the problem was cured because were the ecu temp sensor sits in the coolent bridge the outlet pipe that comes out of the matrix goes back to the coolent bridge the temp sensor was getting proper hat water in stead of look warm getting wrong signal in turn cause low power the car is running great and fuel consumption is better cause the sensor is sending the right signals before the ecu was thinking the car was cold and still giving out loads of fuel how does the heater feel in your car .

yep.. coolant temp sensor is a likely candidate for over fueling..
 
but I still doubt car have other issues..

[cem_devecioglu]

Read the maintenance guides mate

Non dbw Omegas are not OBDII compliant!!

If you don't believe me look here at TBs post #1 http://oldsite.omegaowners.com/forum/YaBB.pl?num=1176839367/0

agreed..

[Marks DTM Calib]

Disconnecting a plug lead on a 2.5V6 won't necessariy throw a fault code

P0300 Random/multiple cylinder misfire detected
P0301 Cylinder 1 Misfire Detected
P0302 Cylinder 2 Misfire Detected
P0303 Cylinder 3 Misfire Detected
P0304 Cylinder 4 Misfire Detected
P0305 Cylinder 5 Misfire Detected
P0306 Cylinder 6 Misfire Detected
No fault will always throw a code but with OBDII misfire detection is pretty good, primary info is from the crank sensor.

Miss fire detection is mostly via the knock sensor based on cam sensor input as to which cylinder is finring next e.g. Fire cylinder 4, fire detected via knock sensor - all good. No firing detected for x consecutive firings then raise code.

The 2.5 and 3.0 do NOT support miss fire detection. 

[dbdb]

misfire detection is via the crank sensor,  the camshaft sensor is used for cylinder identification.

OBD1 codes for GM are
12 System normal
13 oxygen sensor circuit open
14 coolant sensor high resistance or shorted
15 coolant sensor circuit low or open
16 direct ignition system (DIS) fault in circuit
17 cam position sensor fault
18 crank or cam sensor error
19 crank sensor circuit fault
21 tps sensor out of range
22 tps sensor signal voltage low
23 intake air temp sensor out of range, low
24 vehicle speed (vss) sensor circuit fault
25 air temp sensor sensor out of range, high
26 quad-driver module (computer) circuit #1 fault
27 quad-driver module (computer) 2nd gear circuit
28 quad-driver module (computer) circuit #2 fault
29 quad driver module (computer) 4th gear circuit
31 cam position sensor fault
32 egr circuit fault
33 map sensor signal out of range, high
34 map sensor signal out of range, low
35 idle air control sensor circuit fault
36 ignition system circuit error
38 brake input circuit fault
39 clutch input circuit fault
41 cam sensor circuit fault, igntion control circuit fault
42 electronic spark timing (EST) circuit grounded
43 knock sensor
or electronic spark control circuit fault
44 oxygen sensor lean exhaust
45 oxygen sensor rich exhaust
46 pass-key II circuit or ps.pressure switch circuit fault
47 pcm-bcm data circuit
48 misfire diagnosis
51 calibration error, mem-cal, ecm or eeprom failure
52 engine oil temperature circuit, low temperature indicated
53 battery voltage error or egr or pass-key II circuit
54 egr system failure or fuel pump circuit low voltage
55 a/d converter error or pcm not grounded or lean fuel or frounded reference voltage
56 quad-driver module #2 circuit
57 boost control problem
58 vehicle anti-theft system fuel enable circuit
61 a/c system performance or degraded oxygen sensor signal
62 engine oil temperature high temperature indicated
63 oxygen sensor right side circuit open or map sensor out of range
64 oxygen sensor right side lean exhaust indicated
65 oxygen sensor right side rich exhaust indicated
66 a/c pressure sensor circuit low pressure
67 a/c pressure sensor circuit or a/c clutch circuit failure
68 a/c compressor relay circuit failure
69 a/c clutch circuit head pressure high
70 a/c refrigerant pressure circuit high
71 a/c evaporator temperature sensor circuit low
72 gear selector switch circuit
73 a/c evaporator temperature circuit high
75 digital egr #1 solenoid error
76 digital egr #2 solenoid error
77 digital egr #3 solenoid error
79 vehicle speed sensor (vss) circuit signal high
80 vehicle speed sensor (vss) circuit signal low
81 brake input circuit fault
82 ignition control (IC) 3X signal error
85 prom error
86 analog/digital ecm error
87 eeprom error
99 power management